1. Field of the Invention
This invention relates to a structure and method for the fabrication of a III-nitride Vertical Cavity Surface Emitting Laser (VCSEL).
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
One of the main difficulties in producing an (Al,In,Ga)N laser is the formation of Distributed Bragg reflector (DBR) mirrors, particularly the bottom mirror. Two differing approaches to this mirror have been taken: Nichia [1] and Panasonic [2] have used a flip-chip mounting method and then mechanical substrate removal to reveal a smooth facet for the application of a dielectric DBR mirror. This approach has the disadvantage of having very little control over cavity length. This can be alleviated, as demonstrated by Panasonic [2], by using very long cavity length, which results in a narrow longitudinal mode spacing such that there is always a longitudinal mode in alignment with the emission of the quantum well. This, however, makes single longitudinal mode operation impossible.
Another approach is to epitaxially grow a DBR mirror, as demonstrated by T-C Lu et al [3], who used AlN/GaN alternating periods to achieve a high reflectivity mirror. This approach provides precise control of cavity length, but the fabrication of such an epitaxial mirror is extremely difficult due to difficulties like relaxation, and costly due to the raw materials and time needed for such epitaxial growth.
Dielectric mirrors are much simpler to produce, so an approach that allows for precise control of cavity length (such as through epitaxial growth instead of mechanical polishing) with the simultaneous use of dielectric DBR mirrors would be ideal for (Al,In,Ga)N VCSEL fabrication. The present invention satisfies this need.
To date, there have been no public demonstrations of nonpolar or semipolar (Al,In,Ga)N VCSELs. Three groups, including Nichia [1], Panasonic [2], and National Chiao Tung University [3] have demonstrated electrically-injected c-plane (polar) (Al,In,Ga)N VCSELs. These devices have been demonstrated on c-plane GaN or c-plane sapphire and do not exhibit the polarization-locking properties of nonpolar and semipolar VCSELs.